1. Field of the Invention
The present invention relates to interconnection between a power semiconductor element and an electrode for extracting current in the power semiconductor element.
2. Description of the Background Art
In power electronics for driving a motor or the like, an IGBT is characteristically used as a switching device in an area where the rated voltage is 300 V or higher, and a freewheeling diode connected in parallel with the switching device is also used.
An inverter circuit is a dc-ac converter constructed from an IGBT serving as a switching device and a freewheeling diode. The IGBT and freewheeling diode each include 4 or 6 elements for use in motor control. The inverter circuit, having a dc terminal connected to a dc power source, converts a dc voltage to an ac voltage by switching the IGBT, to thereby supply power to a motor which is a load.
In a product of an inverter circuit including six IGBTs and six freewheeling diodes, 6 pairs of one IGBT and one freewheeling diode are arranged on the same substrate. These 6 pairs are formed inside a housing, and this housing has an external collector electrode, an external emitter electrode and an external gate electrode. These external electrodes and each pair of IGBT and freewheeling diode are connected with an aluminum wire. Prior art is disclosed in Japanese Patent application Laid-Open Nos. 9-172136 (1997), 2002-43508 and 2004-228461.
With the development in characteristics of an IGBT and the trend toward shrinkage of the IGBT chip, an IGBT chip these days is half to quarter or smaller in size as compared to that of 10 years ago. On the other hand, since an IGBT and a diode are connected to each other with an aluminum wire in a product of an inverter circuit, the above-described reduction in size of the IGBT chip unavoidably reduces an area of a chip surface region where an aluminum wire should be drawn. However, a problem contradictory to the trend toward smaller chips has recently arisen in that the chip size is determined by the diameter of the aluminum wire since there is a limit imposed on the current value that can be flown through an aluminum wire (that is, the current value is fixed).
Another problem arises in that heat is less likely to escape to the upper portion of a chip since an aluminum wire is connected directly to the chip from above the upper side of the chip.
Further, since the life of heat cycle (H/C) is determined by the area in contact with an aluminum wire, products of recent times have been becoming difficult to satisfy requirements for H/C life expected to have high values.